1. Field of the Invention
The present invention relates to a method for producing an optical waveguide, including the step of forming a polymer having a refractive index difference or distribution from a composition mainly containing two kinds of polymerizable monomers and/or oligomers different in polymerizing mechanism and refractive index by selective polymerization induced by the difference in polymerizing mechanism between the polymerizable monomers and/or oligomers. The invention relates to a method for producing an optical transmission path easily and inexpensively, and a material composition preferably used for the production method. The material composition for producing an optical waveguide and the method for producing an optical waveguide according to the invention can be applied to production of inexpensive low-loss optical waveguide components such as an optical interconnection, an optical demultiplexer and an optical multiplexer in the field of optical fiber communications.
2. Field of the Invention
A technique for forming an optical waveguide device by using a self-condensing phenomenon generated by introduction of beam-like light at a predetermined wavelength into a photo-curable resin mixture solution has attracted public attention.
According to the above production method, first, a predetermined vessel is filled with a mixture solution containing a photo-curable resin of a high refractive index and a photo-curable resin of a low refractive index. Then, in the condition that an end of an optical fiber is immersed in the mixture solution, light in a specific wavelength band capable of curing only the photo-curable resin of the high refractive index is introduced into the other end of the optical fiber. As a result, light is emitted from the immersed end of the optical fiber so that a cured portion of a high refractive index having a diameter equal to the core diameter of the optical fiber can be formed gradually so as to start from the immersed end of the optical fiber by use of a self-condensing phenomenon. Then, light in a predetermined wavelength band is applied on the whole mixture solution so that the two resins in the mixture solution of high and low refractive index photo-curable resins remaining in the vessel are entirely cured. In this manner, a cured portion of a low refractive index is formed around the cured portion of a high refractive index which has been already formed. That is, there is provided a technique for producing an optical waveguide having a stepwise refractive index distribution.
In the above technique, the refractive index is substantially distributed stepwise. To increase the refractive index difference between the core (high refractive index portion) and the clad (low refractive index portion), the core-forming time needs to be elongated to polymerize only the high refractive index material selectively. Hence, production efficiency is not improved. The refractive index in a section of the core is not flat accurately but slightly increases from the center to the periphery. For this reason, the near field pattern of transmission light at an end surface of the optical waveguide exhibits a doughnut-like intensity distribution. Hence, there is a problem that coupling efficiency of such an optical waveguide to a graded index optical fiber having the highest refractive index at the center portion of the core is not good.